Vertical guided probe array providing sideways scrub motion

ABSTRACT

Improved probing of closely spaced contact pads is provided by an array of guided vertical probes that has a sideways scrub relative to the line of contact pads. With this orientation of scrub motion, the probes can be relatively thin along the contact line, and relatively thick perpendicular to the contact line. The thin dimension of the probes allows for probing closely spaced contact pads, while the thick dimension of the probes provides mechanical robustness and current carrying capacity. The probes have a predetermined curvature in a plane including the contact line, to help determine the amount of scrub motion during contact. In a preferred embodiment, an array of probes is provided for probing two closely spaced and parallel rows of contact pads, offset from each other by half the contact pad pitch.

FIELD OF THE INVENTION

This invention relates to making temporary electrical contact to devicesor circuits under test.

BACKGROUND

Testing of electronic devices and circuits prior to final packaging isan important aspect of integrated circuit fabrication, and has beensignificant for some time. Accordingly, methods of making temporaryelectrical contact for such testing have been thoroughly investigated.It is helpful to classify probes for making temporary electrical contactas being either horizontal probes or vertical probes. Horizontal probesare substantially parallel to the chip plane. For example, a horizontalprobe array can be arranged as a set of horizontal needles extendinginward from a frame to make contact to pads at the periphery of an ICchip.

In contrast, vertical probes are substantially perpendicular to the chipplane. For example, in FIG. 1 a, a vertical probe 102 is shown having atip 120 making contact with a contact pad 130, where contact pad 130 isshown in side view. Frequently, as shown here, vertical probes include acurved section. One purpose of the curved section of a vertical probe isto help determine the exact nature of the scrub motion tip 120 makeswith respect to contact pad 130 when contact is made. The ability tofine-tune such scrub motion by appropriate probe design is a significantadvantage of vertical probes relative to horizontal probes. Arrays ofvertical probes, such as 104 on FIG. 1 b, are also known in the art.This figure illustrates another advantage of vertical probes relative tohorizontal probes, namely that arrays of vertical probes can often bemore closely packed than similar arrays of horizontal probes,particularly for 2-D probe arrays.

Vertical probes are referred to as guided probes if the probe assemblyincludes one or more guide plates having slots through which the probespass to control probe motion. In some cases, no guide plates areemployed, and the resulting vertical probes are referred to as “free”(i.e., un-guided) vertical probes. U.S. Pat. No. 5,923,178 is arepresentative prior art example of free vertical probes. U.S. Pat. Nos.4,901,013, 5,952,843, and 5,945,836 are representative examples of priorart guided vertical probe approaches.

As electronic circuit technology progresses, contact pads tend to becomesmaller and more closely spaced, which places increasingly severedemands on probe technology. For example, probing of contact pads spacedby 80 μm or less is presently of increasing interest in manufacturing.As described in greater detail below, conventional vertical probeapproaches tend to fail for such small pitches. Accordingly, it would bean advance in the art to provide vertical probing of contact pads havinga pitch of 80 μm or less.

SUMMARY

Improved probing of closely spaced contact pads is provided by an arrayof guided vertical probes that has a sideways scrub relative to the lineof contact pads. With this orientation of scrub motion, the probes canbe relatively thin along the contact line, and relatively thickperpendicular to the contact line. The thin dimension of the probesallows for probing closely spaced contact pads, while the thickdimension of the probes provides mechanical robustness and currentcarrying capacity. The probes have a predetermined curvature in a planeincluding the contact line, to help determine the amount of scrub motionduring contact. In a preferred embodiment, an array of probes isprovided for probing two closely spaced and parallel rows of contactpads, offset from each other by half the contact pad pitch. Features ofthis preferred embodiment include disposing the tips of each probe closeto the center line of the probe array, and disposing tabs for settingthe vertical position of the probes with respect to an upper guide platesuch that the tabs face away from the center line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a side view of a conventional vertical probe.

FIG. 1 b shows an oblique view of a conventional vertical probe array.

FIG. 1 c shows the relation between contact pad arrangement and scrubdirection for the probe array of FIG. 1 b.

FIGS. 2 a-b show two oblique views of a vertical probe assemblyaccording to an embodiment of the invention.

FIGS. 3 a-d show four orthogonal views of the embodiment of FIGS. 2 a-b.

FIG. 4 shows the relation between contact pad arrangement and scrubdirection for probe arrays according to embodiments of the invention.

FIG. 5 shows a detailed side view of a probe of the embodiment of FIGS.2 a-b.

FIG. 6 shows a multi-layer probe suitable for use in embodiments of theinvention.

DETAILED DESCRIPTION

In order to better appreciate the invention, it is helpful to considersome of the factors that come into play as pitch is decreased in theconventional probe array of FIG. 1 b. These factors can be more clearlyunderstood by reference to FIG. 1 c, which shows an array of contactpads 106, arranged along a line 108, where the probe tip scrub directionis shown as 110. Scrub direction 110 is as shown because of thecurvature of probes 102 in array 104. The scrub motion is generally inthe same plane as the probe curvature, since otherwise the probecurvature has little influence on the scrub motion. Decreasing the pitchof the contact pads requires a corresponding decrease in the probe width(i.e., the probe dimension parallel to line 108). If the probe width isdecreased while holding the probe length (i.e., the probe dimensionparallel to scrub direction 110) constant, there will be an undesirabletendency for the probe to preferentially deform laterally (i.e., alongdirection 108, as opposed to along direction 110) in an uncontrollablemanner, because the probe will be stiffer along direction 110 than alongdirection 108. Note that the curvature of probes 102 in array 104 actsto control probe deformation in direction 110, not direction 108. Suchuncontrollable lateral probe deformation is undesirable because it couldlead to probes incorrectly making contact with neighboring contact pads.

The problem of preferential lateral probe deformation can be avoided bydecreasing the probe length as the probe width is decreased. However,the resulting probe ends up being a thin, wire-like probe, which is hardto handle, mechanically fragile, and cannot readily be designed toprovide a controlled degree of scrub motion. Such a wire probe can beregarded as having an undesirable tendency to deform uncontrollably inany direction.

According to embodiments of the present invention, fine-pitch verticalprobing is provided by probe arrays providing a controlled “sideways”scrub motion. More specifically, such a sideways scrub motion is shownon FIG. 4, where an array of contact pads 402 are arranged along a line406 and the probe scrub direction 404 is parallel to line 406. Thesignificance of the difference between the conventional scrub motiondirection of FIG. 1 c and the sideways scrub of FIG. 4 can be betterappreciated by consideration of the following specific example of apreferred embodiment of the invention.

FIGS. 2 a-b show two oblique views of a vertical probe assemblyaccording to an embodiment of the invention. FIGS. 3 a-d show fourorthogonal views of the embodiment of FIGS. 2 a-b. In this example, avertical probe array 202 is supported by an upper guide plate 204 and byfirst and second lower guide plates 206 and 208 respectively. Two of theprobes are referenced as 202 a and 202 b. Thus this example is a guidedprobe array. The probes pass through slots in the respective guideplates, which serve to control probe position and scrub motion duringcontact. More specifically, the probes can have a feature (e.g., tab 508as shown on FIG. 5) which can engage with upper guide plate 204 forsetting the vertical probe position. Lower guide plates 206 and 208serve to control probe lateral position and to provide control of thescrub motion. In some cases, two lower guide plates are employed asshown. This arrangement is preferred because with two lower guideplates, the channel that guide the probes is increased in length,thereby reducing scrub motion. The invention can also be practiced withone or with three or more lower guide plates. Preferably, any or all ofthe probes of array 202 are replaceable in the probe assembly.

Guide plates 204, 206, and 208 can be made from any material havingsufficient mechanical strength and durability for supporting probe array202. It is important that guide plates 204, 206, and 208 not provideelectrical connection between probes of array 202, so guide plates 204,206, and 208 can be entirely fabricated from electrically insulatingmaterial or can include insulating material sufficient to electricallyisolate the probes from each other.

The key features of this example are best appreciated in the orthogonalviews of FIGS. 3 a-d. It is convenient to define coordinate directionsas follows: the X direction is in the plane of the device under test andis parallel to the line of probe tips; the Y direction is in the planeof the device under test and perpendicular to X; and the Z direction isorthogonal to X and Y. These coordinate axes are shown in the views ofFIGS. 3 c-d.

FIG. 3 b shows a bottom view of the probe array (i.e., looking up towardthe probe tips. From this view, it is apparent that this embodiment issuitable for making contact to two parallel rows of contact pads as onFIG. 4. More specifically, a first set of probes is arranged so thattheir tips are arranged along a first lateral probe array line, and asecond set of probes is arranged so that their tips are arranged along asecond lateral probe array line parallel to the first array line. Inthis example, the probe spacing (i.e., pitch) is the same along thefirst and second array lines, and the two sets of probes are offset fromeach other in the array line direction by half the pitch. Contact padshaving such an arrangement are commonly encountered in practice, andseveral features of this example are advantageous for probing suchstaggered contact pad arrangements.

For example, it is preferred for each probe tip (shown in detail as 512on FIG. 5) to be displaced from the center of its corresponding probetoward a center line 302 of the probe array, as shown on FIG. 3 b. Thisarrangement of the tips facilitates probing closely spaced lines ofcontact pads. Another such feature, best seen on FIG. 3 a, is that eachprobe tab (shown in detail as 508 on FIG. 5) for engaging with upperplate 204 faces away from center line 302. This arrangement of the tabsalso facilitates probing closely spaced lines of contact pads, sinceunnecessary reduction in probe to probe mechanical clearance caused bythe tabs is thereby avoided. For example, it is apparent from FIG. 3 athat if the orientation of the tab on each probe were reversed, theprobe to probe minimum separation would be unnecessarily reduced by thetabs.

A further such feature is that all probes in array 302 curve in the samedirection in the X-Z plane, as best seen in the side view of FIG. 3 c.This arrangement of probe curvature is also preferred for probingclosely spaced contact pads, because it is preferred for all probes toscrub in the same direction. This is important because it facilitatesconsistently aligning scrub marks relative to centers of thecorresponding contact pads. If the probe curvatures were not all in thesame direction, the scrub marks would extend from slot centers inopposite directions, thereby preventing the centering of all scrub markareas to contact pad centers. Since the scrub marks are small,manufacturers frequently like to position the scrub marks consistentlyat a particular location on each pad—not necessarily at the padcenter—creating an undisturbed section of contact pad which is laterused as the wire-bonding zone, because adhesion is better to undisturbedpad surfaces. One consequence of having all probes curve in the samedirection, in combination with the above-described offsets of the tipsand tabs is that array 202 includes two types of probes, which can beregarded as “left” and “right” versions of the same probe. Probes 202 aand 202 b are of-different type, and can be regarded as example of leftand right probes respectively (or vice versa). The end view of FIG. 3 dshows the differences between probes 202 a and 202 b most clearly, whereit should be noted that probes 202 a and 202 b curve in the samedirection, as shown on FIG. 3 c.

FIGS. 3 c and 3 d show a key aspect of this embodiment of the invention.More specifically, the probes of array 202 have a substantiallyrectangular cross section, where the pertinent dimensions are shown asXp and Yp on FIGS. 3 c-d and Xp<Yp. The combination of Xp<Yp and probecurvature in the X-Z plane is basically what provides the controlledsideways scrub as described above. More specifically, by having Xp<Yp,the probes can more easily bend in the X direction than in the Ydirection. By having the probe curvature be in the X-Z plane, this probecurvature helps define and control the scrub motion of the probe tips onthe corresponding contact pads. Finally, the overall mechanicalstability is improved by having the probe length Yp be greater than thewidth Xp, thereby avoiding the above-described problems relating towire-like probes.

Another way to understand this aspect of the invention is as follows.The probe width (i.e., Xp) is constrained to be less than the contactpad spacing or pitch. If Yp is comparable to Xp, then for small Xp theresulting probe is a wire-like probe that is difficult to handle. If Ypis greater than Xp in a conventional vertical probe array (e.g., asshown on FIG. 1 b), then there is an inconsistency in the design—thenatural deflection direction for the probes is in the X direction,because Xp<Yp, but the probe curvature in the Y-Z plane acts to controldeflection in the Y direction. According to the present invention, thisinconsistency is removed by altering the plane of probe curvature fromthe Y-Z plane (as on FIG. 1 b) to the X-Z plane (as on FIG. 3 c). As aresult of this change, the perpendicular scrub of FIG. 1 c is changed tothe sideways scrub of FIG. 4. Briefly, a small Xp allows for probing ofclosely spaced contacts, while a relatively large Yp provides mechanicalrobustness and current carrying capacity.

FIG. 4 shows the relation between contact pad arrangement and scrubdirection for probe arrays according to embodiments of the invention. Asdescribed above, an array 402 of contact pads having a contact line 406is probed by a probe array having a scrub motion 404 parallel to contactline 406. With this scrub motion direction, it is especially importantfor the scrub motion to be small and well-controlled, since an excessivescrub motion can cause a probe to make contact with an incorrect contactpad. For example, in one design, the sideways scrub length is only 8 μm.The conventional probe scrub motion direction of FIG. 1 b has much moreforgiving tolerances on scrub motion, especially since contact padlength (parallel to scrub direction 110) tends to be greater thancontact pad width (parallel to array line 108).

Thus it can be observed that changing from normal scrub (FIG. 1 b) tosideways scrub (FIG. 4) unexpectedly provides a solution to theabove-described design difficulties due to decreasing probe pitch. Itturns out, surprisingly, that the difficulties of controlling scrubmotion sufficiently well to make a sideways scrub viable for closelyspaced contact pads are substantially less than the difficulties facedin making a normal scrub viable for closely spaced contact pads.

FIG. 5 shows a detailed side view of a probe of the embodiment of FIGS.2 a-b. This probe includes a base end 502, a tip end 506 and a shaft 504connecting the base end to the tip end. Probe shaft 504 is curved asdescribed above, while probe base end 502 fits into a slot in upperguide plate 204 and probe tip end 506 fits into slots in lower guideplates 206 and 208. A tab 508 is present on base end 502 to provide avertical position reference relative to upper guide plate 204.Alternatively, it is possible to provide the vertical reference relativeto a lower guide plate, but use of the upper guide plate for verticalreference is preferred. Probe curvature tends to be slight. For example,in one design, a probe having Xp=30 μm and Yp=75 μm has a maximum curveoffset of 75 μm. In the example of FIG. 5, tip end 506 include a tippedestal 510 and a tip 512. Tip pedestal 510 is optional, but ispreferred for probes having a short tip 512 (e.g., on the order of 10 μmhigh). In such cases, the presence of pedestal 510 reduces the chancethat some part of tip end 506 other than tip 512 undesirably makescontact with the device or circuit under test, or with nearby structuressuch as wafer insulation coating etc. Preferably, tip 512 is made of amaterial, such as rhodium, suitable for making reliable temporaryelectrical connections. In some embodiments, base end 502 can also bemade of a material suitable for making temporary electrical connections,and can include an optional base contact point 514. Such embodiments areof interest when temporary connections are formed between the base endsof the probes and the test equipment to which the probe array ismounted.

More generally, base end 502 and tip end 506 can be formed of differentmaterials than shaft 504. Rhodium, hard gold, and palladium are examplesof suitable materials.

FIG. 6 shows a multi-layer probe suitable for use in embodiments of theinvention. In this example, the probe has 5 layers, 602, 604, 606, 608,and 610. Each layer is parallel to the X-Z plane, so lithographicpatterning techniques combined with metal deposition techniques (e.g.,plating) can be employed to precisely determine the curvature of shaft504 in the X-Z plane. Use of multiple probe layers provides a convenientway of defining probe features such as tip 512 (part of layer 604) andtab 508 (i.e., layer 610). Different probe layers can have differentmaterial composition, which is one way to provide different materialcompositions for, e.g., tip 512 and shaft 504. Material composition canalso vary within a layer, e.g., rhodium may only be present in the tippart of layer 604.

The preceding description has been by way of example as opposed tolimitation. In particular, the invention is not limited to probingcontact pads arranged as in FIG. 4. Embodiments of the invention aresuitable for probing any configuration of contact pads including two ormore contact pads in a line. For example, contact pads on the peripheryof a chip can be probed. The invention is also applicable for probingirregular sets of contact pads, e.g., flip chip bumps, provided the setof contact pads includes at least one line of contact pads which can beprobed with a sideways scrub.

1. An apparatus for making electrical contact to a device under testcomprising: a set of two or more probes comprising a multi-layer stack,each probe of said set including a base end, an opposing distal tip end,and a shaft connecting said base end to said tip end; an upper guideplate having slots within which said probe base ends of said set ofprobes are disposed; a first lower guide plate having slots within whichsaid probe tip ends of said set of probes are disposed; and said set ofone or more probes disposed such that said tip ends of said set ofprobes are disposed along a lateral probe array line.
 2. The apparatusof claim 1, wherein, for each said probe, said tip end and said base endhave a different material composition than said shaft.
 3. The apparatusof claim 2, wherein, for each said probe, said tip end and said base endhave material compositions suitable for making temporary electricalcontact via mechanical contact.
 4. The apparatus of claim 1, whereineach said probe is replaceable in said apparatus.
 5. The apparatus ofclaim 1, wherein each said base end of each said probe further comprisesa tab engagable with said upper guide plate for fixing a verticalposition of each said probe.
 6. The apparatus of claim 1, wherein eachsaid tip end of each said probe further comprises a tip for makingelectrical contact to said device under test.
 7. The apparatus of claim1, further comprising a second lower guide plate, within which saidprobe tip ends are disposed.
 8. The apparatus of claim 1, furthercomprising: a second set of two or more probes, each probe of saidsecond set including a base end, a tip end, and a shaft connecting saidbase end to said tip end; wherein said upper guide plate includesopenings within which said base ends of said second set of probes aredisposed; wherein said lower guide plate includes openings within whichsaid tip ends of said second set of probes are disposed; wherein saidsecond set of one or more said probes are disposed such that said tipends of said second set of probes are disposed along a second lateralprobe array line substantially parallel to said first probe array line.9. The apparatus of claim 8, wherein said tip ends of said first set ofprobes are substantially equally spaced by a first pitch, wherein saidtip ends of said second set of probes are substantially equally spacedby a second pitch, wherein said first pitch and said second pitch aresubstantially equal, and wherein said first and second sets of probesare offset from each other by substantially half said first pitch in adirection X.
 10. The apparatus of claim 8, wherein a center line of saidapparatus is parallel to and substantially halfway between said firstand second probe array lines.
 11. The apparatus of claim 10, whereineach said tip end of each said probe of said first and second sets ofprobes further comprises a tip for making electrical contact to thedevice under test, and wherein each said tip is displaced from a centerof its corresponding probe toward said center line, whereby separationbetween said first and second sets of probes can be increased.
 12. Theapparatus of claim 10, wherein each said base end of each said probe ofsaid first and second sets of probes further comprises a tab engagablewith said upper guide plate for fixing a vertical position of each saidprobe, and wherein each said tab is disposed on a side of itscorresponding probe facing away from said center line.
 13. The apparatusof claim 8, wherein each said shaft of each said probe of said first andsecond sets of probes curves in the same direction.
 14. The apparatus ofclaim 1 wherein said set of probes comprises a substantially rectangularcross section.
 15. The apparatus of claim 14 wherein said rectangularcross section of said set of probes comprises a lateral dimension Xp1 ina direction X parallel to said probe array line and has a second lateraldimension Yp1 in a direction Y perpendicular to said probe array line,wherein Xp1<Yp1.
 16. The apparatus of claim 1 wherein said probes aresubstantially vertical.
 17. The apparatus of claim 16 wherein said setof probes are substantially aligned with a vertical direction Zperpendicular to said probe array line, and wherein each said probeshaft of said first set of probes has a predetermined curvature in anX-Z plane.
 18. The apparatus of claim 1 comprising a sideways scrubmotion of said tip ends of said set of probes as contact is made to anarray of contact pads on the device under test.
 19. The apparatus ofclaim 1 wherein said probes comprise metallic layers.
 20. The apparatusof claim 1 wherein at least one said guide plate comprises anelectrically insulated material.
 21. An apparatus for making electricalcontact to a device under test comprising: a set of two or more probescomprising a multi-layer stack, each probe of said set including a baseend, an opposing distal tip end, and a shaft connecting said base end tosaid tip end; an upper guide plate having slots within which said probebase ends of said set of probes are disposed; a first lower guide platehaving slots within which said probe tip ends of said set of probes aredisposed; said set of one or more probes disposed such that said tipends of said set of probes are disposed along a lateral probe arrayline; a second set of two or more probes, each probe of said second setincluding a base end, a tip end, and a shaft connecting said base end tosaid tip end; wherein said upper guide plate includes slots within whichsaid base ends of said second set of probes are disposed; wherein saidlower guide plate includes slots within which said tip ends of saidsecond set of probes are disposed; and wherein said second set of one ormore said probes are disposed such that said tip ends of said second setof probes are disposed along a second lateral probe array linesubstantially parallel to said first probe array line.